Hybrid hard disk drive (HDD) storage devices include one or more rotating magnetic disks combined with nonvolatile solid-state memory. Generally, the nonvolatile solid-state memory includes NAND-based memory cells that provide the ability to access data as quickly as a solid-state drive, while the magnetic disks provide the data storage capacity of a conventional HDD. For this reason hybrid HDDs are expected to be commonly used in laptop computers.
However, one drawback of the NAND-based memory cells in solid-state drives and hybrid HDDs is that the data retention of such cells may be limited by charge leaking from the floating gates of the memory cell transistors. Charge leakage is accelerated by high temperatures and radiation, whose effects are difficult to track. Consequently, data retention time for a particular NAND memory cell is not a fixed, known time period. Instead, there is significant uncertainty in how long a NAND memory cell can reliably retain data. Additional factors compound this uncertainty, including the small manufacturing differences inherent in each NAND memory cell, the damage done to each memory cell by repeated Program-Erase cycles, and variations in the process of programming the data into each NAND cell. Thus, the improved performance provided to a hybrid HDD by NAND-based memory cells is partially offset by the fact that NAND memory cells have limited data retention capability and are inherently less reliable than other storage media in the hybrid HDD.